Differential distribution of olfactory receptor neurons in goldfish: structural and molecular correlates.

The olfactory system of many terrestrial vertebrates comprises a main olfactory organ and a vomeronasal organ each containing a morphologically distinct type of olfactory receptor neuron (ORN). The two cell types also differ in the expression of G-proteins and odorant receptor molecules. Fish do not have a vomeronasal organ, and their olfactory neurons-three different morphological types-are contained in one epithelium. The olfactory organ of goldfish appears as a rosette, with the sensory epithelium lying along the proximal portion of each lamella, where it attaches to the midline raphe. Using immunocytochemistry, in situ hybridization, and electron microscopy, we tested whether a correlation exists between receptor cell morphology, distribution of cell type within the sensory epithelium, and expression of odorant receptors and G-proteins. A strong correlation exists between ORN morphology, type of odorant receptor and G-protein expressed and the distribution of sensory cells within the olfactory epithelium. The Buck and Axel type of odorant receptor and Galpha(olf) are expressed in tall ciliated ORNs distributed homogenously across the entire sensory epithelium. In contrast, microvillous ORNs expressing V2R-like receptors, and Galpha(o), Galpha(q), or Galpha(i-3), and crypt type ORNs expressing Galpha(o) and Galpha(q), are preferentially located along the dorsal margin of the epithelium and near the midline raphe. V2R- and OR-type receptor molecules do not colocalize in one cell, and only crypt-type ORNs express more than one G-protein.

Solitary chemoreceptor cells in the nasal cavity serve as sentinels of respiration.

Inhalation of irritating substances leads to activation of the trigeminal nerve, triggering protective reflexes that include apnea or sneezing. Receptors for trigeminal irritants are generally assumed to be located exclusively on free nerve endings within the nasal epithelium, requiring that trigeminal irritants diffuse through the junctional barrier at the epithelial surface to activate receptors. We find, in both rats and mice, an extensive population of chemosensory cells that reach the surface of the nasal epithelium and form synaptic contacts with trigeminal afferent nerve fibers. These chemosensory cells express T2R "bitter-taste" receptors and alpha-gustducin, a G protein involved in chemosensory transduction. Functional studies indicate that bitter substances applied to the nasal epithelium activate the trigeminal nerve and evoke changes in respiratory rate. By extending to the surface of the nasal epithelium, these chemosensory cells serve to expand the repertoire of compounds that can activate trigeminal protective reflexes. The trigeminal chemoreceptor cells are likely to be remnants of the phylogenetically ancient population of solitary chemoreceptor cells found in the epithelium of all anamniote aquatic vertebrates.